Conventionally, a thin film such as silicon oxide (SiO2) or silicon nitride (Si3N4 or the like) has been used widely for a semiconductor device such as a thin film transistor, a photoelectric converting device or the like. The following three kinds of methods are mainly used as a method of forming the thin film such as the silicon oxide or the silicon nitride.
(1) Physical Vapor Phase Film Forming Method Such as Sputtering or Vacuum Deposition
More specifically, in the method, a solid thin film material is set to be an atom or an atomic group by a physical technique and is deposited over a surface on which a film is to be formed, and a thin film is thus formed.
(2) Thermal CVD Method
More specifically, in the method, the thin film material of a gas is set to have a high temperature, to form a thin film by chemical reaction.
(3) Plasma CVD method
More specifically, in the method, the thin film material of a gas is changed into a plasma to form a thin film by chemical reaction.
In particular, the plasma CVD method (plasma enhanced chemical vapour deposition) in (3) has been used widely because a dense and uniform thin film can be efficiently formed (see Japanese Laid-Open Patent Publication No. Hei 9-69504 and Japanese Laid-Open Patent Publication No. 2001-28362).
A plasma CVD apparatus 100 to be used in the plasma CVD method is generally constituted as shown in FIG. 15.
More specifically, the plasma CVD apparatus 100 comprises a reaction chamber 102 maintained under reduced pressure, and an upper electrode 104 and a lower electrode 106 are provided to be opposed to each other at a constant interval in the reaction chamber 102. A film forming gas supply path 108 connected to a film forming gas source which is not shown is connected to the upper electrode 104 in such a manner that a film forming gas is supplied into the reaction chamber 102 through the upper electrode 104.
Moreover, a high frequency applying device 110 for applying a high frequency is connected to the vicinity of the upper electrode 104 in the reaction chamber 102. Furthermore, an exhaust path 114 for discharging an exhaust gas through a pump 112 is connected to the reaction chamber 102.
In the plasma CVD apparatus 100 thus constituted, monosilane (SiH4), N2O, N2, O2, Ar or the like in the formation of the film of the silicon oxide (SiO2) and monosilane (SiH4), NH3, N2, O2, Ar or the like in the formation of the film of the silicon nitride (Si3N4 or the like) are introduced through the film forming gas supply path 108 and the upper electrode 104 into the reaction chamber 102 maintained in a pressure, for example, 130 Pa.
In this case, for example, a power having a high frequency of 13.56 MHz is applied through the high frequency applying device 110 to a portion between the electrodes 104 and 106 provided opposite to each other in the reaction chamber 102, thereby generating a high frequency electric field. In the electric field, an electron is caused to collide with the neutral molecule of a film forming gas so that a high frequency plasma is formed and the film forming gas is decomposed into an ion and a radical.
By the action of the ion and the radical, a thin silicon film is formed on the surface of a semiconductor product W such as a silicon wafer which is provided on the lower electrode 106 to be one of the electrodes.
In such a plasma CVD apparatus 100, a thin film material such as SiO2 or Si3N4 is also stuck and deposited onto the surface of an internal wall, an electrode or the like in the reaction chamber 102 not only the semiconductor product W on which a film is to be formed. By a discharge in the reaction chamber 102, a by-product is formed in a film forming process.
The by-product is peeled by a dead weight, a stress or the like when it grows to have a constant thickness, and particulates are mixed as foreign matters into a semiconductor product, thereby causing a contamination in the film forming process. Thus, a thin film of high quality cannot be manufactured so that the disconnection or short circuit of a semiconductor circuit might be caused, and furthermore, a yield or the like might also be deteriorated.
For this reason, conventionally, such by-product is removed after the end of the film forming process in the plasma CVD apparatus 100 by using a cleaning gas, a fluorine containing compound such as CF4, C2F6 or COF2 and O2 or the like are added if necessary, for example (see the Japanese Laid-Open Patent Publication No. Hei 9-69504 and the Japanese Laid-Open Patent Publication No. 2001-28362).
More specifically, in the conventional cleaning method for the plasma CVD apparatus 100 using such a cleaning gas, a cleaning gas constituted by a fluorine containing compound such as CF4, C2F6 or COF2 is introduced in place of a film forming gas in the film formation together with a gas such as O2 and/or Ar through the film forming gas supply path 108 and the upper electrode 104 into the reaction chamber 102 maintained under reduced pressure after the film forming process is ended as shown in FIG. 15.
In the same manner as in the film formation, a high frequency power is applied through the high frequency applying device 110 to a portion between the electrodes 104 and 106 provided opposite to each other in the reaction chamber 102, thereby generating a high frequency electric field, and an electron in the electric field is caused to collide with the neutral molecule of the cleaning gas to form a high frequency plasma so that the cleaning gas is decomposed into an ion and a radical.
Then, the ion and the radical react to a by-product such as SiO2 or Si3N4 which is stuck and deposited onto the surface of the internal wall, the electrode or the like in the reaction chamber 102 so that the by-product is changed into a gas as SiF4. Consequently, the SiF4 is discharged together with an exhaust gas to the outside of the reaction chamber 102 through the exhaust path 114 by means of the pump 112.
In such cleaning, under the existing circumstances, a time required for the cleaning is determined experientially to end the cleaning.
If the time required for the cleaning is too short, accordingly, the by-product such as SiO2 or Si3N4 stuck and deposited onto the surface of the internal wall, the electrode or the like in the reaction chamber 102 remains. When the CVD apparatus is used repetitively, consequently, particulates are mixed into a semiconductor product, resulting in a contamination as described above. Therefore, a thin film of high quality cannot be manufactured so that the disconnection or short circuit of a semiconductor circuit is caused, and furthermore, a yield or the like is also deteriorated.
To the contrary, if the time required for the cleaning is too long, the cleaning gas introduced into the reaction chamber 102 is exactly discharged without contributing to the cleaning. For this reason, a cleaning gas constituted by a fluorine containing compound such as CF4 or C2F6 is discharged into the air. Consequently, there is a possibility that an environment might be adversely influenced.
More specifically, the fluorine containing compound such as CF4 or C2F6 to be used as a cleaning gas has a long lifetime and is stable in the air. Moreover, there is a problem in that a gas discharging process is hard to perform after the cleaning and the cost of the process is high. Furthermore, a global warming potential (an integral period 100-year value) is very great, that is, 6500 with CF4, 9200 with C2F6 and 23900 with SF6 so that the global warming is greatly influenced.
If the time required for the cleaning is thus prolonged, moreover, a throughput is reduced so that a productivity is deteriorated.